1. Field of The Invention
The present invention relates to the processing of cathode ray tubes and particularly to the processing of such tubes which have been previously processed by flashing of a barium-containing getter in the tube, and were reworked and now require a second processing including a second flashing of the getter.
Cathode ray tubes, whether of the monochrome or color variety, basically comprise a cathodoluminescent screen and at least one electron beam generating means contained within an evacuated glass envelope. The envelope is usually funnel-shaped, the expansive portion having a closed end in the form of a face panel with the cathodoluminescent screen disposed relative to the interior surface of the panel. Within the neck portion of the funnel, one or more electron guns are oriented in a manner to beam electrons accurately to the screen. A typical electron gun includes an electron emissive source aligned with the apertures of several axially spaced grid electrodes which control, accelerate and focus the emitted electron beam. Usually, a sealed exhaust tubulation projects externally from the closed end of the neck portion as does an array of metallic pins effecting external electrical connections for the several internal tube components.
There are occasions when a cathode ray tube is found to contain a malfunctioning or defective component, for example, an electron gun. Such an occasion may arise during tube manufacture or as a result of tube operation. It is then conventional practice to repair the tube by removing the defective gun and replacing it with a new one. Essentially, this practice involves breaking the vacuum of the tube, for example, with ambient air, removing the defective gun by cutting off a portion of the neck, sealing a new neck portion thereto and sealing a replacement gun into the renecked tube. The regunned tube may then be normally processed in the same manner as virgin tubes. This normal processing includes, in the sequence recited, the general steps of exhausting and baking the tube to outgas the internal tube components, sealing the tube to form a closed vacuum vessel, gettering, aging and then high-voltage conditioning the tube. The processed tube is finally subjected to the standard operating tests (raster, dot, etc.).
While testing such conventionally processed regunned cathode ray tubes, approximately 70%-80% manifest an undesireable condition, namely, a dark-centered cathode. Investigation has revealed that this dark center is a carbon layer which has deposited on the electron emissive surface of the replacement cathode. This carbon layer can reduce the cathode electron emission.
2. Description of The Prior Art
After normally baking/exhausting the tube to a vacuum, and then tube sealing, it has been usual practice to getter and then to age the sealed tube. Gettering the tube further reduces the pressure within the tube envelope. Aging the tube stabilizes the electron emissive characteristics of the cathode.
Gettering the tube involves using getters containing barium which is flashed or vaporized within the tube. The vaporized getter material, barium, sorbs the residual gases within the tube and removes them as low vapor-pressure solid condensates. Also, the barium film which is deposited on the internal surfaces of the tube continues to sorb gases which are liberated during normal tube operation.
During tube or, more particularly, cathode aging, the cathode is heated and electron currents not less than normal operating currents are drawn from the cathode to other various grid electrodes for a time sufficient to stabilize the cathode emission. This aging step also beneficially removes contaminants from the electron bombarded grid electrodes while liberating trapped gases in and around them. Some of these gases are adsorbed by the barium film while other formerly trapped gases become part of the tube's internal atmosphere.
The grid electrodes of a virgin or regunned tube commonly may then be subjected to high-voltage potential differences which burn off additional undesired contaminants, points, particles, etc. from the electrodes before the tube is tested. As is well known, such high-voltage conditioning or "spot knocking" inhibits the occurrence of inter-electrode arcing during normal tube operation.
Significantly, the conventional step sequence of getter flashing before cathode aging and tube conditioning, heretofore, has been held inviolate by the industry. There has been a universally held belief that an operational vacuum, that is a vacuum approximately equal to that existing during normal tube operation, was necessary for proper aging or conditioning. Accordingly, getter flashing, which acts as a chemical pump to decrease the pressure within the tube, has been performed before the other steps. Further, it was believed that flashing in any other position of the manufacturing cycle could degrade the beneficial results of aging the cathode or high-voltage conditioning the tube by over flashing the getter.
This conventional processing for regunned cathode ray tubes has resulted in high tube scrap levels caused by dark-centered cathodes. In fact, approximately 15% of all regunned tubes exhibiting a dark-centered cathode must be scrapped. This is wasteful. The remaining approximately 85% of such tubes exhibit an impaired focus quality during operation.
Because a regunned tube has been completely processed once before during its original manufacture, including flashing the barium getter, the original barium film becomes oxidized and reacts with moisture to form hydrates when the original gun is removed from the neck, for example, by cracking the neck glass. Exhausting and baking the tube at approximately 400.degree. C. which precedes the second getter flashing does not remove the waters of hydration. However, while flashing the second getter, temperatures sufficient to decompose the hydrates are attained. The moisture so released reacts with metal carbides already present within the tube to produce hydrocarbon gases. The substantial electron activity, including secondary emission, which occurs in and around the gun area of the tube during cathode aging ionizes these hydrocarbon gases into various positive ions including those bearing carbon. These carbon bearing ions, presumably, are not adsorbed by the second getter film before they are attracted to the nearby highly-negative cathode. Upon decomposition, the ions deposit as carbon on the electron emitting surface of the cathode to form the dark center.
Prior art attempts to avoid this carbon center have proven less than satisfactory. For example, in U.S. Pat. No. 4,048,545, the original barium film is treated with a gas mixture consisting of CO.sub.2 and water vapor soon after cracking the tube neck glass. Theoretically, the original barium film is converted to stable barium carbonate and the H.sub.2 O from the barium hydrates is removed before the tube is regunned and conventionally processed. In practice, however, drawbacks still remain because the prior art treatment continues to result in a substantial number of regunned tubes exhibiting a dark-centered cathode. Possibly, there is not a complete conversion of the original barium film to the relatively stable carbonate. Also, the prior art method disadvantageously requires additional gases and their associated gas handling systems both of which increase the commercial expense of processing regunned tubes.